You are an adult and as such, you can do what you like in relationships. However, you’re also a parent, so considering how major changes will affect the kids, such as having a partner move in, is very important.
If you’ve come to the point where a new partner is going to move in, you need to talk to your kids about it first. Try to do so with just you and them and not with the new partner present. This will give them the opportunity to be as honest as possible without worrying about hurting the new partner’s feelings.
What’s more, if you and your ex are on reasonable speaking terms, you should try to keep them in the loop about what’s going on with your new partner moving in. The kids will talk to your ex about the new partner and the best scenario is that both you and your ex are on the same page with how you’re going to handle things.
If you and your ex aren’t amicable enough to have a reasonable conversation about this, you’re going to have to do your best to positively support your kids in all this and if possible, reach out for some help from a counsellor or other neutral supportive person.